Using jenkins 2.19.2 (or 2.19.4, through official docker image), with envinject, Environment Injector Plugin 1.93.1.
Doing an empty "freestyle" job build and on the result cheking out the environment-Variables page (i.e. http://192.168.99.100:8080/job/test/2/injectedEnvVars/) will not show the "Workspace" variable.
Also not included when using the REST-API (i.e. http://192.168.99.100:8080/job/test/2/injectedEnvVars/api/json?pretty=true).

It IS however set (tested via "execute shell" -> printvars).

After activating the "Inject environment variables to the build process" in the job configuration (even without setting anything), the Workspace variable shows up in the results.